In North America and elsewhere, it is common that large numbers of homes subscribe to a CATV to receive by coaxial cable a number of commercial and public television signals. Each signal consists of television video and audio, comprised of continuous audio and 30 (or 25) video frames per second. Each signal is transmitted down the coaxial cable by the CATV system to the subscriber's home television set over a discrete band of frequency known as a "channel". The subscriber tunes his television to the channel desired and receives the composite television signal.
It is not uncommon that a CATV have upwards of 50,000 subscribers. On such a large CATV, there is generally a central "hub" which acts as a control center to receive the various signals, whether "broadcast" or satellite delivered, or locally generated by CATV operators or third parties. These signals are put on a trunk coaxial cable. Signal losses occur along the CATV coaxial cables and it is thus common that amplifiers, known in the trade as bridger amplifiers, be located at strategic locations, or nodes. Typically therefore, a CATV system comprises a plurality of antennas adapted to receive signals from a plurality of broadcast stations or from satellite transponders. Amplifiers and the like located at a central hub amplify these signals and transmit each of them as a unique channel of frequencies onto one or more trunk coaxial cables Along the trunk coaxial cable are a plurality of nodes. At each node a bridger amplifier amplifies the signals from the trunk and conveys the amplified signals onto one of a plurality of secondary distribution coaxial cables. Each secondary distribution cable has a plurality of "taps", each of which has a plurality of "drop" coaxial cables, one to each subscriber. Generally, any secondary distribution cable will service approximately 200 subscribers and there are normally between 50 and 100 taps so that each tap will service 2 to 4 drops.
Other systems have been developed to interactively distribute information to subscribers' televisions at their request. In order to be viable, such systems must:
1. service a large number of subscribers simultaneously; PA0 2. delivery high quality video images within a short response time; PA0 3. operate without requiring new hardware in the subscriber's home; and PA0 4. operate within the constraints of the number of television channels available on a typical CATV. Most CATV's have a maximum of approximately 15 "empty" channels. PA0 1. CATV: a community antenna television system as presently in use in North America. PA0 2. Subscriber: a user of the CATV or other television transmission system who receives the television signals at his television set in his home. PA0 3. Video frame: the combination of two interleaved video fields, each such field being composed of (1) a plurality of scan lines called the vertical blanking interval (defined below) and (2) a second larger plurality of scan lines containing the video data to be converted to a video image on the television screen. Video frames are transmitted to the CATV at 30 frames/second (North American and Japanese standard) or 25 frames/second (European standard). The invention will be described according to the North American standard transmission rate of 30 frames/second, but operates in the same way at 25 frames/second. A single video frame presents a video-quality still image. PA0 4. Vertical blanking interval: the first 21 lines of a video field, containing coded information to synchronize the presentation of the video image. Several of the lines in the vertical blanking interval are presently left blank and are used in the invention to insert addressing information. PA0 5. Trunk cable: the primary coaxial distribution cable from a CATV central hub. PA0 6. Node: points along a trunk cable at which bridger amplifiers amplify the television signal and split it for transmission down secondary distribution coaxial cables. PA0 7. Distribution cable: a secondary distribution coaxial cable emanating from a node. PA0 8. Tap: a point on a distribution cable where the television signal is split and sent down drop coaxial cables to the subscribers home. PA0 9. Drop: a drop coaxial cable to a subscribers home. PA0 10. Node frame store: a device, located within a multiple node frame store at a node, that can receive and store a video frame and retransmit that frame 30 times/second down the distribution cable. The node frame store also receives the audio message associated with the video frame and transmits this audio in synchronization with the appropriate video frame, both on the same television channel, down its distribution cable. PA0 11. Multiple node frame store: a group of node frame stores, all located at one node, each servicing its unique distribution cable. PA0 (A) a cable hub for receiving, generating and amplifying a plurality of television signals that are broadcast over predetermined frequency channels; PA0 (B) a trunk cable with one end connected to the cable hub to receive and transmit said signals to; PA0 (C) a plurality of nodes along said cable, each node adapted to convey said signals to; PA0 (D) a bridger amplifier connected to the trunk cable at each node and adapted to amplify and to transmit said signals onto; PA0 (E) a distribution cable for transmitting said amplified signals through; PA0 (F) a tap to a drop cable to a subscriber's home that is adapted for connection to a subscriber's television receiver; an improved system selectively delivering pre-recorded video frames and audio messages, wherein a video frame consists of at least one video field being a first plurality of scan lines representing a video blanking interval, and a second plurality of scan lines representing video picture data; PA0 (G) a central control unit (CPU), located at the cable hub, comprising; PA0 (1) selecting PA0 (2) assigning a unique address to the secondary path; PA0 (3) inserting upon a predetermined scan line of the vertical blanking interval of each fram a unique address; PA0 (4) transmitting the video frame with the unique address along the primary path; PA0 (5) examining, on the primary path at the node, the predetermined scan line of the video frame for the unique address; PA0 (6) storing that video frame at the node if the unique address matches the address of the secondary path; and, PA0 (7) repetitively transmitting, at least 25 times/second, the video frame stored at the node down the secondary path to the subscriber's television set as a still video frame. PA0 (1) means for examining a primary path for a video frame that has one of the scan lines in its video blanking interval uniquely addressed; and PA0 (2) means for storing a video frame and for transmitting the stored video frame repetitively at least 25 times per second onto a second path; PA0 (3) comparator means for comparing the address examined by means (1) and on a match to a predetermined address, activating the storing means (2) to store the video frame from the primary path.
No interactive system to date has succeeded in meeting all of the above requirements. Typically, the number of simultaneous users has been limited to a very small number. To send a live video image, one television channel is required for each subscriber; therefore, only approximately 15 subscribers could be serviced at one time. Or, in order to increase the number of simultaneous users, only computer graphics have been delivered, sacrificing picture quality and speed of delivery. The invention described in this application meets all four of the above-mentioned requirements. In addition, the invention delivers accompanying audio messages along with the video images, for a complete presentation.